Antiferromagnetically stabilized pseudo spin valve for memory applications

ABSTRACT

The invention relates to improving the switching reliability of a magnetic memory cell in a magnetic random access memory (MRAM). Embodiments of the invention add an antiferromagnet to a magnetic memory cell. An antiferromagnetic layer can be formed adjacent to a soft layer in the MRAM on a side of the soft layer that is opposite to a hard layer of the MRAM. One embodiment further includes an additional interlayer of non-antiferromagnetic material between the antiferromagnetic layer and the soft layer.

RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/760,127, filed Jan. 16, 2004 [Attorney Docket: MICRON.219C1],which is a continuation application of U.S. application Ser. No.10/193,458, filed Jul. 10, 2002 [Attorney Docket: MICRON.219A ], nowU.S. Pat. No. 6,707,084, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 60/354,623 [Attorney Docket:MICRON.219PR ], filed Feb. 6, 2002, the disclosures of which are herebyincorporated by reference in their entireties herein.

This application is also related to U.S. application Ser. No.11/103,347, filed Apr. 11, 2005 [Attorney Docket: MICRON.219C1DV1], thedisclosure of which is hereby incorporated by reference in its entiretyherein.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract NumberMDA972-98-C-0021 awarded by DARPA. The Government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to memory technology. In particular, theinvention relates to non-volatile magnetic memory.

2. Description of the Related Art

Computers and other digital systems use memory to store programs anddata. A common form of memory is random access memory (RAM). Many memorydevices, such as dynamic random access memory (DRAM) devices and staticrandom access memory (SRAM) devices are volatile memories. A volatilememory loses its data when power is removed. For example, when aconventional personal computer is powered off, the volatile memory isreloaded through a boot up process. In addition, certain volatilememories such as DRAM devices require periodic refresh cycles to retaintheir data even when power is continuously supplied.

In contrast to the potential loss of data encountered in volatile memorydevices, nonvolatile memory devices retain data for long periods of timewhen power is removed. Examples of nonvolatile memory devices includeread only memory (ROM), programmable read only memory (PROM), erasablePROM (EPROM), electrically erasable PROM (EEPROM), flash memory, and thelike. Disadvantageously, conventional nonvolatile memories arerelatively large, slow, and expensive. Further, conventional nonvolatilememories are relatively limited in write cycle capability and typicallycan only be programmed to store data about 10,000 times in a particularmemory location. This prevents a conventional non-volatile memorydevice, such as a flash memory device, from being used as generalpurpose memory.

An alternative memory device is known as magnetoresistive random accessmemory (MRAM). An MRAM device uses magnetic orientations to retain datain its memory cells. Advantageously, MRAM devices are relatively fast,are nonvolatile, consume relatively little power, and do not suffer froma write cycle limitation. A pseudo spin valve (PSV) MRAM device uses anasymmetric sandwich of the ferromagnetic layers and metallic layer as amemory cell, and the ferromagnetic layers do not switch at the sametime.

The asymmetric sandwich of a PSV MRAM includes a “hard layer” thatstores data and a “soft layer” that switches or flips to allow data tobe stored and read in the hard layer. When operating as intended, thesoft layer switches before the hard layer. The earlier switching of thesoft layer advantageously inhibits switching of the hard layer, whichthen results in a higher write threshold for a PSV MRAM than for a spinvalve MRAM.

One problem with conventional PSV MRAM devices is that the magnetizationof the soft layer is not well controlled. A soft layer that fails toswitch at a relatively low applied magnetic field can result in a PSVMRAM device that undesirably behaves as a spin valve rather than a PSV.This reduces the write threshold and can result in corrupting the storeddata during a read operation. To protect PSV MRAM devices from datacorruption, the fields generated during read operations are maintainedto relatively low levels, which results in relatively low repeatabilityand cyclability of writing to and reading from memory cells.

SUMMARY OF THE INVENTION

Embodiments of the invention solve these and other problems bystabilizing the soft layer of a pseudo spin valve (PSV). Embodiments ofthe invention include a layer of antiferromagnetic material (AFM), whichstabilizes the magnetization of the thin layer. The stabilization of thesoft layer of the PSV provides PSV MRAM devices with relatively goodrepeatability and cyclability.

Embodiments of the invention include an antiferromagnet in a magneticmemory cell. An antiferromagnetic layer can be formed adjacent to a softlayer in an MRAM on a side of the soft layer that is opposite to a hardlayer of the MRAM. One arrangement further includes an additionalinterlayer of non-antiferromagnetic material between theantiferromagnetic layer and the soft layer.

The antiferromagnetic material (AFM) is formed adjacent to or near tothe soft layer of the PSV. The layer of AFM should be formed on a sideof the soft layer that is opposite to a side with a hard layer of thePSV. In addition, an amount of coupling between the soft layer and theAFM layer should be sufficiently low enough to allow the soft layer toswitch at a lower magnetic field than the hard layer, therebymaintaining a relatively wide spread between the strength of a magneticfield used in a read operation and the strength of a magnetic field usedin a write operation.

One embodiment of the invention includes an antiferromagneticallystabilized pseudo spin valve (ASPSV) in a magnetic random access memory(MRAM). The ASPSV includes a hard layer of ferromagnetic material, asoft layer of ferromagnetic material, a spacer layer ofnon-ferromagnetic material disposed between the hard layer and the softlayer; and an antiferromagnetic layer disposed adjacent to the softlayer. The antiferromagnetic layer should also be disposed on a side ofthe soft layer that is opposite to the hard layer. The antiferromagneticlayer can be formed from an alloy of manganese, such as from ferromanganese (FeMn).

Another embodiment of the invention includes an antiferromagneticallystabilized pseudo spin valve (ASPSV) in a magnetic random access memory(MRAM) with an AFM interlayer. The ASPSV includes a hard layer offerromagnetic material adapted to store data in a magnetic orientation,a soft layer of ferromagnetic material adapted to switch orientation toallow data to be read from the hard layer, a spacer layer ofnon-ferromagnetic material disposed between the hard layer and the softlayer, an antiferromagnetic layer disposed on a side of the soft layerthat is opposite to the hard layer, and the AFM interlayer. The AFMinterlayer is disposed between the soft layer and the antiferromagneticlayer. The AFM interlayer can be formed from a variety of materials, butshould not be formed from an antiferromagnetic material. Suitablematerials for the AFM interlayer include iridium (Ir), copper (Cu),ruthenium (Ru), chromium (Cr), and aluminum (Al). The AFM interlayer canbe relatively thin, such as about a monolayer in thickness.

Another embodiment of the invention includes a method of stabilizing apseudo spin valve (PSV). The method includes providing amagnetoresistive sandwich that includes a soft layer and a hard layer,and forming an antiferromagnetic layer on the magnetoresistive sandwichnear to the soft layer and on a side of the soft layer that is oppositeto the hard layer. The antiferromagnetic layer can be formed adjacent tothe soft layer, or a AFM interlayer can also be formed between the softlayer and the antiferromagnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will now be described withreference to the drawings summarized below. These drawings and theassociated description are provided to illustrate preferred embodimentsof the invention and are not intended to limit the scope of theinvention.

FIG. 1 is a perspective view illustrating a giant magneto-resistance(GMR) cell in a spin valve mode.

FIG. 2 is a schematic top-down view illustrating an array of GMR cells.

FIG. 3 illustrates a GMR cell in a pseudo spin valve (PSV) mode.

FIG. 4 is a cross-sectional view of a magnetoresistive stack for anantiferromagnetically stabilized pseudo spin valve (ASPSV) according toan embodiment of the invention.

FIG. 5 is a cross-sectional view of a magnetoresistive stack for anASPSV according to another embodiment of the invention.

FIG. 6 is an R-H plot of an ASPSV illustrating thresholds for writingdata when the ASPSV is not selected.

FIG. 7 is an R-H plot of an ASPSV for a write operation, where the ASPSVis subjected to the presence of a digital field.

FIG. 8 is an R-H plot of an ASPSV illustrating thresholds for writingdata when the ASPSV is selected.

FIG. 9 is an R-H plot of an ASPSV illustrating thresholds for readingdata when the ASPSV is not selected.

FIG. 10 is an R-H plot of an ASPSV illustrating thresholds for readingdata when the ASPSV is selected.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although this invention will be described in terms of certain preferredembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thebenefits and features set forth herein, are also within the scope ofthis invention. Accordingly, the scope of the invention is defined onlyby reference to the appended claims.

A magnetoresistive random access memory (MRAM) stores data in magneticstates of its memory cells. The electrical resistance of the cell variesdepending on the stored magnetic state of the cell. The stored state ofthe cell is detected by sensing the difference in resistance.

FIG. 1 is a perspective view illustrating a GMR cell 100 in a spin valvemode. The GMR cell 100 includes a word line 102 and a bit line 104. In aGMR cell, the bit line 104 is also known as a sense line. The bit line104 contains magnetic layers. Data is stored in a cell body portion ofthe bit line 104 by simultaneously applying current through the wordline 102 and the bit line 104. The direction of the currents in the wordline 102 and in the bit line 104 (and the consequent magnetic fieldapplied) determines the polarization of the magnetic orientation thatstores the logical state of the data. For example, the applied fieldcomponent from the bit line current can be clockwise around the bit line104 for a first current direction, and counterclockwise around the bitline 104 for a second current direction, and similarly for the word line102. The vector sum of the applied magnetic fields from the two (ormore) conductive lines determines the magnetic state of the cell.

To read data from the GMR cell 100, currents are again applied to theword line 102 and the bit line 104 corresponding to the GMR cell 100.The resistance encountered by the current applied to the bit line 104varies depending on the logical state stored in the magnetic layers. Acell with a larger resistance exhibits a larger voltage drop with thecurrent than a cell with a smaller resistance.

FIG. 2 is a schematic top-down view illustrating an array 200 of GMRcells. A plurality of cells are arranged into the array 200 in a memorydevice. The array 200 of cells includes a plurality of word lines 202and a plurality of bit lines 204. An individual cell within the array200 is selected by applying current through the corresponding word lineand the corresponding bit line. Data is not stored or read in a cellwhere current flows through only one of the cell's word line or bitline.

FIG. 3 illustrates a GMR cell 300 in a pseudo spin valve (PSV) mode. TheGMR cell 300 includes a word line 302 and a bit line 304. The bit line304 of the GMR cell 300, which is also known as a sense line, furtherincludes a GMR stack including a first magnetic layer 306, a conductivelayer 308, and a second magnetic layer 310. The first magnetic layer 306and the second magnetic layer 310 are mismatched so that the firstmagnetic layer 306 is magnetically “softer” than the second magneticlayer 310. The mismatch in magnetic properties can be obtained by makingthe first magnetic layer 306 relatively thin as compared to the secondmagnetic layer 310; by selecting a relatively soft magnetic material forthe first magnetic layer 306 and a relatively hard magnetic material forthe second magnetic layer 310; or both. Other terms used to describe a“hard layer” include “pinned layer” and “fixed layer.” However, it willbe understood by one of ordinary skill in the art that the storedmagnetic orientation in a hard layer can be varied in accordance withthe logical state of the stored data. Other terms used to describe a“soft layer” include “variable layer” and “flipped layer.” It will beunderstood by one of ordinary skill in the art that the GMR stack canfurther include multiple layers of ferromagnetic materials and spacers.

The GMR cell 300 stores data as a magnetic orientation in the secondmagnetic layer 310. A relatively high magnetic field is required toswitch the magnetization of the second magnetic layer 310 so that themagnetization remains fixed in operation. The magnetic state of the GMRcell 300 is switched by switching the magnetization of the firstmagnetic layer 306, which can be switched with a relatively low magneticfield generated by applying current to the corresponding word line 302and the corresponding bit line 304. The resulting magnetization of thefirst magnetic layer 306 is either parallel or anti-parallel to themagnetization of the second magnetic layer 310. When the magnetizationin the first magnetic layer 306 is parallel with the magnetization ofthe second magnetic layer 310, the electrical resistance of the GMR cell300 is lower than when the magnetization is relatively is anti-parallel.Current in the word line 302 and/or the bit line 304 can be switched inboth directions to correspondingly switch the magnetization of the firstmagnetic layer 306, i.e., the soft magnetic layer, between parallel andanti-parallel states. The difference in electrical resistance of the bitline 304 is then sensed, thereby allowing the stored logical state ofthe GMR cell 300 to be retrieved.

FIG. 4 illustrates a cross-sectional view of a magnetoresistive stack400 for an antiferromagnetically stabilized PSV cell according to anembodiment of the invention. Test results for the magnetoresistive stack400 will be described later in connection with FIGS. 6 through 10.Although the magnetoresistive stack 400 is shown with barrier or caplayers and with extra interlayers, it will be understood by one ofordinary skill in the art that embodiments of the invention includethose without all the layers described herein. For example, barrierlayers can be selected depending on the fabrication processes used andon the composition of the substrate assembly, insulating layers,conductors, and the magnetoresistive materials themselves.

The illustrated magnetoresistive stack 400 includes an underlayer 402, ahard layer 404, a first interlayer 406, a spacer layer 408, a secondinterlayer 410, a soft layer 412, an antiferromagnetic (AFM) layer 414,a first cap layer 416, and a second cap layer 418. The underlayer 402 orseeding layer provides adhesion between an underlying layer in thesubstrate and the hard layer 404 by providing texture to the stack. Theunderlayer 402 can also protect against the undesired diffusion of atomsfrom the hard layer 404 to an underlying layer, such as a siliconsubstrate. A variety of materials can be used for the underlayer 402. Inone embodiment, the underlayer 402 is formed from tantalum (Ta). Othermaterials that can be used for the underlayer 402 include titanium (Ti),ruthenium (Ru), nickel iron chromium (NiFeCr), and tantalum nitride(TaN). The underlayer 402 can be formed to a broad range of thicknesses.In one embodiment, the underlayer 402 is within a range of about 10Angstroms (Å) to about 100 Å thick. Various processing techniques, suchas physical vapor deposition (PVD) techniques, chemical vapor deposition(CVD) techniques, and the like, can be used to form the various layersdescribed herein.

The hard layer 404 (or thick layer) stores the data for theantiferromagnetically stabilized PSV cell. A relatively large wordcurrent, which generates a relatively large magnetic field, switches theorientation of the magnetic moment stored in the hard layer 404 to storedata. The hard layer 404 can be made from a variety of ferromagneticmaterials, such as permalloy (Ni₈₀Fe₂₀), cobalt-iron (Co₉₀Fe₁₀), and thelike. In one embodiment, the hard layer 404 is within a range of about20 Å to about 100 Å thick.

The first interlayer 406 is optional. The first interlayer 406 can beincluded in the magnetoresistive stack 400 to enhance the signal, i.e.,the change in resistance, from the magnetoresistive stack 400. In oneembodiment, where the hard layer 404 is formed from permalloy, the firstinterlayer 406 is formed from cobalt (Co) or from an alloy that includescobalt, such as Co₉₀Fe₁₀, Co₈₀Fe₂₀, and the like. In one example, thethickness of the first interlayer 406 is within a range of about 2 Å toabout 15 Å.

The spacer layer 408 is a nonmagnetic layer that separates the magneticlayers. The spacer layer 408 can be formed from a broad variety ofnon-ferromagnetic materials. A broad variety of materials can be used toform the spacer layer 408. In one embodiment, the spacer layer 408 iscopper (Cu). Alloys of copper are also suitable materials, such ascopper silver (CuAg), copper gold silver (CuAuAg), and the like. In oneexample, the thickness of the spacer layer 408 is within a range ofabout 18 Å to about 45 Å.

The second interlayer 410 is optional. The second interlayer 410 can beincluded to enhance the signal from the magnetoresistive stack 400. Inone embodiment, where the soft layer 412 is formed from permalloy, thesecond interlayer 410 is formed from cobalt (Co) or from an alloy thatincludes cobalt, such as Co₉₀Fe₁₀, Co₈₀Fe₂₀, and the like. The thicknessof the second interlayer 410 can correspond to a range of about 2 Å toabout 15 Å.

The magnetic moment of the soft layer 412 (or thin layer) can beswitched or flipped with relatively low word currents and relatively lowmagnetic fields. When the magnetic moment of the soft layer 412 and themagnetic moment of the hard layer 404 are parallel, the resistance ofthe PSV cell is relatively low. When the magnetic moment of the softlayer 412 and the magnetic moment of the hard layer 404 areanti-parallel, the resistance of the PSV cell is relatively high. Thesoft layer 412 can be made from a variety of materials, such aspermalloy (Ni₈₀Fe₂₀), cobalt-iron (Co₉₀Fe₁₀), and the like. In oneembodiment, the thickness of the soft layer 412 is about 20% to about80% thinner than the thickness of the hard layer 404.

The AFM layer 414 is a layer of an antiferromagnetic material. Anantiferromagnetic material produces anti-parallel alignments of electronspins in response to an applied magnetic field and has no net magneticmoment. The AFM layer 414 assists to control the magnetization of thesoft layer 412 such that the soft layer 412 more consistently switchesmagnetic moments at a relatively low applied magnetic field, therebyallowing the antiferromagnetically stabilized PSV MRAM to maintainrelatively safe and robust high write thresholds, i.e., improves theswitching reliability of the thin layer. Electrically, the AFM layer 414is in parallel with the soft layer 412. This can disturb the detectionof the variable resistance from the soft layer 412, which is used todetect the memory state stored in the hard layer 404. To reduce thedisturbance to the detection of the memory state, the AFM layer 414should be formed from a material with a relatively high resistivityand/or should be relatively thin.

The AFM layer 414 is preferably formed from an alloy of manganese, suchas an antiferromagnetic alloy of ferro manganese (FeMn) includingFe₅₀Mn₅₀. Other suitable alloys of manganese include iridium manganese(Ir₂₀Mn₈₀), platinum manganese (PtMn), and nickel manganese (Ni₄₅Mn₅₅).The AFM layer 414 can also be formed from an oxide of a ferromagneticmaterial, such as nickel oxide (NiO) and nickel cobalt oxide (NiCoO),and the like, but such oxides can be relatively unstable overtemperature. The AFM layer 414 should be disposed on a side of the softlayer 412 that is opposite to the hard layer 404. In addition, the AFMlayer 414 should not be so thick that pinning of the soft layer results,which detrimentally results in spin valve characteristics from thepseudo spin valve. In one embodiment, the thickness of the AFM layer 414is within a range of about 10 Å to about 70 Å. The thickness of the AFMlayer 414 can vary according to the thickness and the switching fieldsof the hard layer 404 and the soft layer 412.

The first cap layer 416 (or protective cap layer) provides adhesion tothe AFM layer 414 and provides a barrier against the undesired diffusionof atoms from the AFM layer 414 to other layers in the substrateassembly. In one embodiment, the first cap layer 416 is formed fromtantalum (Ta). Other materials that can be used for the first cap layer416 include copper (Cu), titanium nitride (TaN), and the like. Thethickness of the first cap layer 416 can vary in a broad range. In oneembodiment, the thickness of the first cap layer 416 is within about 50Å to about 500 Å thick.

The second cap layer 418 (or diffusion barrier cap layer) is an optionallayer. For some etching processes, the addition of the second cap layer418 provides a relatively good stopping layer. In one embodiment, thesecond cap layer 418 is a layer of chromium silicon (CrSi). Othermaterials that can be used for the second cap layer 418 include copper(Cu), tantalum (Ta), titanium nitride (TiN), and the like. In oneembodiment, the thickness of the second cap layer 418 is within a rangeof about 100 Å to about 200 Å thick, but it will be understood by one ofordinary skill in the art that the thickness can vary within a broadrange.

FIG. 5 illustrates a cross-sectional view of a magnetoresistive stack500 for an antiferromagnetically stabilized pseudo spin valve accordingto another embodiment of the invention. The magnetoresistive stack 500includes the underlayer 402, the hard layer 404, the first interlayer406, the spacer layer 408, the second interlayer 410, the soft layer412, the first cap layer 416, and the second cap layer 418 describedearlier in connection with FIG. 4. In addition, the magnetoresistivestack 500 includes an AFM interlayer 502 and an AFM layer 504, which aredisposed between the soft layer 412 and the first cap layer 416. Unlikethe AFM layer 504, the AFM interlayer 502 is not formed from anantiferromagnetic material. In one embodiment, the AFM interlayer 502 isformed from a relatively thin layer of iridium (Ir). Other suitablematerials for the AFM interlayer 502 include copper (Cu), ruthenium(Ru), chromium (Cr), aluminum (Al), and others.

The AFM layer 504 can be formed from an antiferromagnetic material, suchas ferro manganese (FeMn). Other materials that are suitable for the AFMlayer 504 include various other alloys of manganese as well as variousoxides as described earlier in connection with FIG. 4. The AFM layer 504should be disposed on a side of the soft layer 412 that is opposite tothe hard layer 404. In one embodiment, the thickness of the AFM layer504 is within a range of about 70 Å to about 200 Å. The increase inthickness of the AFM layer 504 (versus the AFM layer 414) increases theexchange coupling such that the ferromagnetic film on the other side ofthe AFM interlayer 502 is affected.

The AFM interlayer 502 is disposed between the AFM layer 504 and thesoft layer 412. In one example, the AFM interlayer 502 has a thicknesswithin a range of about 1 Å to about 5 Å. Preferably, the AFM interlayer502 is about a monolayer in thickness, i.e., about one atomic layerthick. In one embodiment, the AFM interlayer 502 is less than amonolayer in thickness. The AFM interlayer 502 serves as a spacer layerbetween the AFM layer 504 and the soft layer 412.

Advantageously, the AFM interlayer 502 can be used to adjust or toselect the amount of coupling between the AFM layer 504 and the softlayer 412 by reducing the coupling strength between the AFM layer 504and the hard layer 404 and/or the soft layer 412. However, the AFMinterlayer 502 should not be so thick that coupling between the AFMlayer 504 and the soft layer 412 is lost. Further advantageously, theAFM interlayer 502 can also enhance the uniformity of the couplingbetween the AFM interlayer 502 and the soft layer 412.

FIGS. 6-10 are R-H test plots of an example of the antiferromagneticallystabilized pseudo spin valve (ASPSV) described earlier in connectionwith FIG. 4. It will be understood by one of ordinary skill in the artthat the test results will vary substantially in accordance with aselection of materials, layer thicknesses, and cell geometries. In FIGS.6-10, a vertical axis, i.e., the y-axis, corresponds to resistance andhas units of ohms as indicated to the far right of FIGS. 6-10. To thefar left of FIGS. 6-10, the resistance is also indicated as a percentagechange based on the minimum resistance shown for the respective figure.A horizontal axis, i.e., the x-axis, indicates magnetic field strengthand has units of oersteds (Oe).

FIG. 6 is an R-H plot taken from an example of the magnetoresistivestack 400 described earlier in connection with FIG. 4. The R-H plot ofFIG. 6 illustrates the resistance of the magnetoresistive stack 400versus a first magnetic field (“H-field”) that is swept along one axisof the magnetoresistive stack 400. The applied first H-field isrepresented along a horizontal or x-axis of FIG. 6. No other H-field isapplied to the magnetoresistive stack 400, so that the data in FIG. 6 isrepresentative of the conditions that the magnetoresistive stack 400would encounter in operation when the corresponding ASPSV cell is notselected. Bold data lines correspond to data taken with the firstH-field swept in one direction, termed a forward direction; and thindata lines correspond to data taken with the first H-field swept in theopposite direction, termed a reverse direction.

As illustrated in FIG. 6, the magnetoresistive stack 400 advantageouslydoes not switch until the magnitude of the first H-field has reachedabout 75-80 Oe, which is relatively high. This indicates that ASPSVcells that are not selected can tolerate a relatively high H-fieldwithout losing data.

FIG. 7 is an R-H plot of the example of the magnetoresistive stack 400described earlier in connection with FIGS. 4 and 6. The R-H plot of FIG.7 again illustrates the resistance of the magnetoresistive stack 400versus the first H-field. However, a second H-field that isapproximately orthogonal to the first H-field is also applied to themagnetoresistive stack 400 for the data shown in FIG. 7. The secondH-field approximates the H-field that would be generated by a currentflowing through a conductor that is used to select the ASPSV cell withthe magnetoresistive stack 400 from an array of ASPSV cells in an MRAM.This second H-field is sometimes referred to in the art as a “digital”field.

The horizontal or x-axis represents the first H-field that is sweptalong one axis of the magnetoresistive stack 400. When themagnetoresistive stack 400 is subjected to the second H-field, themagnetoresistive stack 400 switches for a write when the magnitude ofthe first H-field is about 53 Oe. This is lower than the approximately75-80 Oe described in connection with FIG. 6, and indicates that a writeto a selected ASPSV cell can occur without undesirably overwriting thecontents of an ASPSV cell that was not selected.

FIG. 8 is an R-H plot of the example of the magnetoresistive stack 400described earlier in connection with FIGS. 4, 6, and 7. The R-H plot ofFIG. 8 illustrates the resistance of the magnetoresistive stack 400versus the first H-field. However the magnetoresistive stack 400 is alsoexposed to another H-field, termed a third H-field. The third H-field isgenerated by passing a current flowing through a select line, such as aword line or a bit line, that is used to select the ASPSV cell thatcorresponds to the magnetoresistive stack 400. This allows an individualASPSV cell to be selected from an array of ASPSV cells in an MRAM. Withthe current flowing through the select line, the magnetoresistive stack400 switches for a write when the magnitude of the first H-field isabout 50 Oe. Thus, FIGS. 6 and 8 illustrate that the magnetoresistivestack 400 can store data when selected with a first H-field strength ofonly about 50 Oe, and yet retain the data when not selected until thefirst H-field strength reaches about 75-80 Oe, thereby providing marginfor the safe writing of data.

FIG. 9 is an R-H plot of the example of the magnetoresistive stack 400described earlier in connection with FIGS. 4, 6, 7, and 8. The R-H plotof FIG. 9 illustrates the resistance of the magnetoresistive stack 400versus an H-field that is swept along one axis of the magnetoresistivestack 400. In FIG. 9, no other H field is applied to themagnetoresistive stack 400, so that the data plotted in FIG. 9illustrates the magnitude of H-field that permits a read from themagnetoresistive stack 400 when the magnetoresistive stack 400 is notselected, i.e., not subject to the H-field of an activated select line,such as a bit line or a word line that is carrying current. In theexample shown in FIG. 9, the magnetoresistive stack 400 tolerates anH-field of a relatively high magnitude of about 70 Oe before themagnetoresistive stack 400 destabilizes and changes resistance. Thechange of resistance is undesirable at relatively low H-field strengthsbecause an ASPSV cell that has not been selected should not changeresistance. An unintended change in resistance can disadvantageouslycorrupt the reading of the resistance of the intended or selected ASPSVcell.

FIG. 10 is an R-H plot of the example of the magnetoresistive stack 400described earlier in connection with FIGS. 4, 6, 7, 8, and 9. The R-Hplot of FIG. 10 illustrates the resistance of the magnetoresistive stack400 versus an H-field that is swept along one axis of themagnetoresistive stack 400. Another H-field generated from a select linethat is carrying a current, such as a word current or a bit current in aword line or in a sense line, respectively, is also applied to themagnetoresistive stack 400. It will be apparent that the scale of thex-axis of the R-H plot of FIG. 10 varies from the R-H plots described inconnection with FIGS. 6, 7, 8, and 9.

The magnetic orientation of the soft layer 412 of the magnetoresistivestack 400 is switched or flipped in both directions, and the differencein resistance is interrogated to read the value of the data stored inthe magnetoresistive stack 400. The R-H plot of FIG. 10 illustrates thatthe soft layer 412 of the magnetoresistive stack 400 can be switched orflipped for a read at an advantageously low H-field strength of about 35Oe. The H-field read separation between the magnetoresistive stack 400when it has been selected and the magnetoresistive stack 400 when it hasnot been selected is about 35 Oe. This advantageously allows acorresponding MRAM to use a relatively broad range of H-field strengthsto read from individual cells without risking the corruption of dataduring the read.

Various embodiments of the invention have been described above. Althoughthis invention has been described with reference to these specificembodiments, the descriptions are intended to be illustrative of theinvention and are not intended to be limiting. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined in theappended claims.

1. A digital system comprising: a magnetic random access memory (MRAM)configured to store data in antiferromagnetically stabilized pseudo spinvalves (ASPSVs), where an ASPSV further comprises: a hard layer offerromagnetic material, where the hard layer is adapted to store data ina magnetic orientation; a spacer layer of non-ferromagnetic materialdisposed adjacent the hard layer; a soft layer of ferromagnetic materialdisposed adjacent the spacer layer such that the spacer layer is betweenthe hard layer and the soft layer, where the soft layer is adapted toswitch magnetic orientation to allow data to be read from the hardlayer; an antiferromagnetic layer disposed on a side of the soft layerthat is opposite to the spacer layer; and an AFM interlayer disposedbetween the soft layer and the antiferromagnetic layer, where the AFMinterlayer is not formed from an antiferromagnetic material.
 2. Thedigital system as defined in claim 1, wherein the AFM interlayer isabout 1 Å to about 5 Å in thickness.
 3. The digital system as defined inclaim 1, wherein the AFM interlayer is about a monolayer in thickness.4. The digital system as defined in claim 1, wherein the AFM interlayeris less than a monolayer in thickness.
 5. The digital system as definedin claim 1, wherein the antiferromagnetic layer comprises an alloy ofmanganese.
 6. The digital system as defined in claim 1, wherein theantiferromagnetic layer comprises at least one of nickel oxide (NiO) andnickel cobalt oxide (NiCoO).
 7. The digital system as defined in claim1, wherein the AFM interlayer comprises iridium (Ir).
 8. The digitalsystem as defined in claim 1, wherein the AFM interlayer comprises atleast one of copper (Cu), ruthenium (Ru), chromium (Cr), and aluminum(Al).
 9. The digital system as defined in claim 1, wherein the spacerlayer is in direct contact with the hard layer, where the soft layer isin direct contact with the spacer layer, where the AFM interlayer is indirect contact with the soft layer, and where the antiferromagneticlayer is in direct contact with the AFM interlayer.